1. Field of the Invention
This invention relates generally to improvements in methods and apparatus for uniformly implanting a stent and, more particularly, to improved uniform stent implantation systems wherein radial expansion is controlled.
2. Description of Related Art
In typical percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient through the brachial or femoral arteries and advanced through the patient's vasculature until the distal end of the guiding catheter is in the ostium of the desired coronary artery. A guidewire and a dilatation catheter having a balloon on the distal end of the dilatation catheter are introduced through the guiding catheter with the guidewire sliding within the dilatation catheter. First, the guidewire is passed through the guiding catheter and into the patient's coronary vasculature. Second, the dilatation catheter is advanced over the previously passed guidewire until the dilatation balloon is properly positioned across a lesion. Once in position across the lesion, a preformed balloon carried by the catheter is inflated to a predetermined size with a liquid at relatively high pressures (e.g., greater than about 4 atmospheres) to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile, so that the dilatation catheter can be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery. The PTCA procedure is also typically performed with the use of a guiding catheter, wherein a conventional over-the-wire system is employed.
In such angioplasty procedures, there may be restenosis of the artery, which necessitates either another angioplasty procedure, a surgical bypass operation, or some other method of repairing or strengthening the dilated area. To assist in the prevention of restenosis and to strengthen the dilated area, a physician can implant an intravascular prothesis, generally called a stent, to maintain vascular patency inside the artery at the site of the lesion. Stents are also used to repair vessels having a flap or dissection or to generally strengthen a weakened section of a vessel. The stent is expanded to a larger diameter, often by the balloon portion of the dilatation catheter. Stents delivered to a restricted coronary artery, expanded to a larger diameter by a balloon catheter, and left in place within the artery at the site of the dilated lesion are shown, for example, in U.S. Pat. No. 4,740,207 (Kreamer) and U.S. Pat. No. 5,007,926 (Derbyshire).
Although stents have been used effectively for some time, the effectiveness of a stent can be diminished if it is not uniformly implanted within the artery. For example, balloons having a stent placed upon them tend to have non-uniform radial expansion due to the increased restriction the stent imposes on the working length of the balloon. Consequently, the balloon expands first at the proximal and distal balloon ends along the path of least resistance, i.e., towards the distal and proximal ends of the balloon, which expands the balloon in a "dog bone" fashion lacking uniform radial expansion. Thus, when the balloon expands in this "dog bone" fashion, the proximal and distal regions of the balloon over expand to form a characteristic "dog bone" shape, the stent is not expanded uniformly and the stent may be improperly implanted.
Accordingly, those concerned with the design, development, and use of stent implantation systems have long recognized the desirability and need for further improvements in systems for uniformly implanting a stent in order to maximize stent performance. In this regard, what has been needed and, heretofore unavailable, is a stent delivery system which controls the radial expansion of the stent along its entire length to ensure uniform expansion.